Mobile communication systems have primarily been developed to provide voice communication services while guaranteeing user mobility. Mobile communication systems have gradually extended the communication service area thereof to high-speed data communication services in addition to voice communication services. However, due to a lack of resources and a demand for higher-speed communication services from users in currently available mobile communication systems, an enhanced mobile communication system is required.
To meet such requirements, Long Term Evolution (LTE), as one next-generation mobile communication system under development, is being standardized in the 3rd Generation Partnership Project (3GPP). LTE is technology for implementing high-speed packet-based communication at a transfer rate of up to about 100 Mbps. Several methods are being discussed to implement high-speed packet-based communication. Examples of such methods include reducing the number of nodes on a communication path by simplifying the architecture of a network and maximally approximating radio protocols to radio channels.
FIG. 1 is a view illustrating the structure of a typical LTE mobile communication system.
Referring to FIG. 1, a radio access network of an LTE mobile communication system may include next-generation base stations (Evolved Node Bs, EUTRAN, hereinafter referred to as eNBs or Node Bs) 110, a Mobility Management Entity (MME) 120 and a Serving Gateway (S-GW) 130.
A piece of user equipment (hereinafter, referred to as a UE) 100 accesses an external network via an eNB, the S-GW, and a P-GW (PDN Gateway (Packet Data Network Gateway)).
The eNB 110, which is a Radio Access Network (RAN) node, corresponds to the Radio Network Controller (RNC) of the Universal Terrestrial Radio Access Network (UTRAN) system and the Base Station Controller (BSC) of the GSM EDGE Radio Access Network (GERAN) system. The eNB 110 connects to the UE 100 over a radio channel and performs a function similar to that of the legacy RNC/BSC. The eNB may use several cells at the same time.
In LTE, not only real-time services such as VoIP (Voice over IP) using the Internet protocol, but also all user traffic are provided over a shared channel. Accordingly, a device to perform scheduling by collecting status information of UEs is needed, and the eNB serves as this device.
The MME 120 is responsible for various control functions. One MME may connect to multiple eNBs.
The S-GW 130, which is a device for providing a data bearer, is controlled by the MME 120 to generate or delete a data bearer.
The core network of the LTE mobile communication system may further include nodes such as an Application Function, a PCRF and a P-GW (not shown) in addition to the MME 120 and the S-GW 130.
The Application Function (AF) is a device for exchanging application-related information with a user at the application level.
The PCRF (Policy Charging and Rules Function) is a device for controlling a policy associated with the Quality of Service (QoS) for the user, and a PCC (Policy and Charging Control) rule corresponding to the policy is delivered to the P-GW and applied. The PCRF (Policy Charging and Rules Function) is an entity for comprehensively controlling QoS and charging for traffic.
A path connecting the UE 100 to a RAN node 110, the RAN node 110 to the S-GW 130, and the S-GW 130 to a P-GW 160, through which user data is transmitted/received is generally referred to as a user plane (UP). In a UP path, a link between the UE 100 and the RAN node 110 uses a radio channel subjected to heavy resource restriction.
In a wireless communication system such as LTE, QoS may be applied in units of Evolved Packet System (EPS) bearers. One EPS bearer is used to transport IP flows having the same QoS requirements. QoS-related parameters may be designated for the EPS bearer. The QoS-related parameters include a QoS Class Identifier (QCI) and an Allocation and Retention Priority (ARP). The QCI is a parameter that defines a QoS priority with an integer, and the ARP is a parameter to determine whether to allow or reject generation of a new EPS bearer.
The EPS bearer corresponds to a Packet Data Protocol (PDP) context in a General Packet Radio Service (GPRS) system. One EPS bearer belongs to a PDN connection, which may have an Access Point Name (APN) as an attribute. To generate a PDN connection for an IP Multimedia Subsystem (IMS) service such as Voice over LTE (VoLTE), a well-known IMS APN needs to be used.
To support voice calls in the LTE network, IMS-based VoLTE may be used according to a Packet Switched (PS) scheme, or CS fall back (CSFB) reusing a Circuit Switched (CS) scheme of the 2G/3G system may be used. In the LTE network, VoLTE may be a term having the same concept as Voice over IMS (VoIMS).
In a wireless communication system, particularly, the LTE system, when an incoming or outgoing voice call occurs while a UE is using an LTE network, a CSFB process of switching to a Circuit Switched (CS) network is performed for a voice service. In this case, a separate authentication procedure for the UE needs to be performed, and thus delay may occur in providing the voice service. In general, the 2G/3G system is a CS network capable of providing a CS service, and an entity responsible for a control operation related to the CS service is referred to as an MSC or VLR. In LTE, CSFB providing a switching function for the CS service is performed between the MSC/VLR and the MME by utilizing an SGs interface. If congestion occurs in a Radio Access Network (RAN), the occurrence is preferably signaled to a Core Network (CN) to perform further optimized traffic processing.
To apply traffic control for each UE, bearer or service according to congestion, the CN needs to recognize UEs located in a RAN in which congestion is occurring.